THE SPINAL CORD. 409 



limited to a small extent of the posterior columns it does not usually 

 affect the voluntary movements ; but if it extend for a distance of several 

 centimetres, in either the cervical or the dorso-lumbar region, it always 

 causes a disturbance of these movements; and when it occupies the 

 whole length and thickness of the posterior columns, the patient can 

 neither stand nor walk, although while lying down and with the aid 

 of vision he can still move his limbs in any direction. 



But the sclerosis of the posterior columns producing locomotor 

 ataxia is confined to their lateral portions. In this instance the disease 

 is not a secondary degeneration, but a primary alteration of structure 

 in the nervous tract, involving more or less completely its various parts. 

 According to Charcot, degeneration or sclerosis of the columns of Goll 

 (Fig. 108) never produces ataxia ; while sclerosis of the lateral parts 

 of the posterior columns (Fig. 109) is always accompanied by ataxic 

 symptoms, and these symptoms are more marked on the right or left 

 side or in the upper or lower limbs, according to the seat of the 

 structural alteration. 



These facts all point to the existence in the spinal cord of a power of 

 reflex muscular coordination, dependent for its exercise on the longi- 

 tudinal fibres of the posterior columns. 



Another important action of the spinal cord, as a nervous centre, is 

 its control over the sphincters and the muscles of evacuation. 



While the small intestine, the caecum, and the colon are supplied 

 exclusively with nerves from the sympathetic system, the lower portion 

 of the rectum receives branches from the sacral plexus of spinal nerves, 

 distributed both to its mucous membrane and its muscular layer. 

 The lower part of the large intestine is in great measure a temporary 

 reservoir, in which the feces accumulate until the time arrives for their 

 evacuation. The rectum, however, is in general nearly empty till 

 shortly before evacuation ; and when the feces begin to pass into it 

 from above, it is still capable of retaining them for a certain period. 

 Their retention and discharge are provided for by two sets of mus- 

 cular fibres ; namely, first, the sphincter ani, which keeps the orifice of 

 the anus closed ; and, secondly, the levator ani and the circular fibres 

 of the rectum, which by their contraction open the anus and expel the 

 feces. Both these acts are regulated by the reflex influence of the 

 spinal cord. 



In the normal condition, the sphincter ani is habitually contracted, 

 thus preventing the escape of the contents of the intestine. An external 

 irritation, applied to the verge of the anus, causes increased contraction 

 and more complete occlusion of its orifice. This habitual closure of 

 the sphincter, which is a purely involuntary act, as efficient during sleep 

 as in the waking condition, depends on the reflex action of the spinal 

 cord. 



But when the rectum has become distended to a certain point, the 

 nervous action changes. The impression then conveyed to the spinal 

 cord causes relaxation of the sphincter ani. At the same time the 



